向dhtmlxlayout写入html代码，header伸缩时标题改变

效果图如下：
源码：
<!DOCTYPE html>
<html>
<head>
    <title>Collapsed text</title>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
    <meta http-equiv="X-UA-Compatible" content="IE=edge"/>
    <link rel="stylesheet" type="text/css" href="../../../codebase/dhtmlx.css"/>
    <script src="../../../codebase/dhtmlx.js"></script>
    <style>
        div#layoutObj {
            position: relative;
            margin-top: 20px;
            margin-left: 20px;
             600px;
            height: 400px;
        }
    </style>
    <script>
        var myLayout;
        var sel;
        function doOnLoad() {
            myLayout = new dhtmlXLayoutObject({
                parent: "layoutObj",
                pattern: "3L",
                cells: [
                    {id: "a", text: "Main Page", collapsed_text: "Expand to view main page"},
                    {id: "b", text: "Site Navigation", collapsed_text: "Expand to navigae"},
                    {id: "c", text: "Support & Feedback"}
                ]
            });
            myLayout.cells("a").attachHTMLString("Custom collapsed text here");
            myLayout.cells("b").attachHTMLString("Custom collapsed text here");
            myLayout.cells("c").attachHTMLString("Collapsed text not changed here");
        }
    </script>
</head>
<body onload="doOnLoad();">
    <div id="layoutObj"></div>
</body>
</html>